Charging a flat battery off the alternator!

This is something I've always done when jump starting cars.

When started, I check for 13.5+v across the terminals to be certain the alternator's working.

However, I've had a look at the RACQ website and its advising the following -

After leaving the car's lights on and flattening the battery, going for a drive will recharge it

You won’t fully recharge your battery by going for a drive, idling the engine, or going for a short stop-start trip. In fact, ‘surface’ charging or continuous undercharging will lower the capacity of the battery over time and shorten its life. You could also void the battery warranty by not recharging it correctly.
The only way to reliably restore a flat battery’s charge is to use an appropriate multi-stage battery charger. The charger voltage needs to be high enough to mix the battery acid evenly in the electrolyte to prevent ‘stratification’.

I've never had bother with the alternator charging the battery!
I agree with you, the RACQ website is nonsense. Early alternators output about 30 amps, and nowadays 55, 75 or even more is typical. If that won't recharge it, why would a charger? Built-in regulators control the voltage to 13.6-14 volts, giving 2-3 amp trickle charge when fully charged. Modern systems controlled by the engine management computer I think reduce the trickle charge to near zero, saving a little power and perhaps giving better battery life by lower gassing.
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As I indicated above (without technical knowledge but relying on logic) if it were true then no car on the road, that has never had the battery charged by a charger, will have a fully charged battery.

Is there some mythical value of fully charged which is over and above that required for satisfactory operation?

If so, it obviously doesn't matter so what is the point of it? If not, then it cannot be correct.
A fully charged battery should theoretically read 2.2 volts per cell on an open circuit, if a battery is continuously charged much in excess of that, as on a battery charger it will result in degradation of the cells mainly due to loss of electrolyte.

I have a couple of 12v 100ah batteries as standby for my ham radio equipment, bought them secondhand 10 or more years ago, I set the charging voltage at 13.2 volts continuous and they are still functioning very well. Car batteries on the other hand need to charge quickly so a higher voltage is needed but they won't last as long.

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I can see some sense in the RAC idea, years ago in the days of the dynamo we were told to get a good battery life it needed an equalising charge every so often, over time some cells become more discharged than others, so once every couple of months trickle charging over night got the cells equal again.

When the alternator first came in there was a rush to swap old cars to alternators, however unless pulley was also changed there was no benefit, the idea was the alternator can spin faster than a dynamo so it still charges at tick over, however the charge rate was very low, there were specials like the CAV AC203 used on coaches and buses that needed current as well as voltage control that could give a good output at tick over, but most were not that good.

And with the alternator the voltage dropped, typical was 13.8 volt, however dynamo two bobbin regulators were set to 16 volt open circuit, so the alternator needed the battery to stop it going over voltage as charging was mark/space controlled not gradual, but the battery was only really used to start engine, once started the alternator supplied all needs.

To float charge any battery takes a long time, first 70% is reasonable quick, so a jump start and soon battery will start car, but to fully charge it takes days, as already said the Lidi smart battery charger will charge at 3.8A to start with, then 3A and then 0.8A and finally 0.1A until it switches off completely which only really happens with small VRLA batteries a 75 Ah battery will never likely switch off, it shows fully charged as the charge sequence reaches the 0.1A stage. And once it leaves the 3.8A stage it is charging just as quick in fact a lot quicker than a car alternator.

Now I don't tend to charge charge car battery, but I do charge the caravan battery, so typical arrive home and use motor mover to get into back garden, then remove battery and use Lidi charger, since I have used an energy meter which plugs in to socket, I have been able to monitor charge rates. So 3.8A less than 15 minutes, the 3A for around an hour, then 0.8A for around 16 hours before it drops to 0.1A. Remember before motor mover used it was maintained at 13.8 volt by caravan charger.

So it takes 24 hours to fully recharge with a terminal voltage of 14.4 volts. With a lower voltage it will take longer, does not matter how much the cars alternator will deliver it's the time a battery takes to absorb that charge, so if we assume an hour a day driving, then by end of month the battery will have likely recovered, but even then not as fully as it would have done with a Lidi charger over night, but unlikely to cause problems.

However modern car does not fully recharge battery anyway, so likely it would start to harden the sulphur on the plates before it is recharged. Years ago we thought we were doing well if a battery lasted 3 years, today before the smart alternator we have been looking at 7 to 10 years, so if not recharging the battery has damaged it, likely it will be years before it fails anyway so we as the user will be unaware of the damage.

So yes I think RAC is correct, in fact I think every two months we should use a smart charger to charge battery like we did in old days of dynamo, it will not hurt the battery, and could well extend the life by a few years, however in real terms we will never know.